1. Technical Field of the Invention
The present invention relates to a picture/audio coding apparatus wherein the coding is executed by determining a distribution of quantities of codes allocated to the picture and the audio signal.
2. Description of the Prior Art
Recently, multi-media communications have been developed greatly, wherein moving pictures are handled, as well as text data, audio data, and graphics data. For the multi-media communications, the integrated service digital network (ISDN) has been spread widely. Further, TV phone and TV conference are implemented, due to high speed modems in public switched telephone network (PSTN). Particularly, picture signal and audio signals are multiplexed for transmission in the TV phone system in the digital or analogue networks. The communication protocols and coding formats according to the kinds of communication networks are provided by the recommendations of ITU-T (International Telecommunication Unionxe2x80x94Telecommunication Standardization Sector).
A block diagram of a conventional picture/audio encoding apparatus which is used in the ISDN, PSTN, and mobile communication networks is shown in FIG. 66. As shown in FIG. 66, picture encoder 1 encodes inputted picture signal according to the variable length coding formats such as H.261 or H.263 provided by the ITU-T recommendations, in order to output the coded picture signal toward buffer 2. Audio encoder 3 encodes inputted audio signal according to the formats such as AD-PCM ( adaptive-differential pulse code modulation) or CELP (code-exited linear predictive coding), in order to output the coded audio signal toward buffer 4. Buffer 2 stores temporarily the coded picture signal from picture encoder 1 and then outputs them toward multiplexer 5, while buffer 4 stores temporarily the coded audio signal from audio encoder 3 and then outputs them toward multiplexer 5. Finally, multiplexer 5 mutiplexes the coded picture signal outputted from buffer 2 and the coded audio signal outputted from buffer 4, in order to output the combined bit stream onto a non-shown transmission channel. Picture encoder 1 monitors the code quantity of in buffer 2 for controlling the code quantity generated in the encoding process in picture encoder 1.
A block diagram of a conventional picture/audio decoding apparatus is shown in FIG. 67. As shown in FIG. 67, demultiplexer 6 separates the combined bit stream from picture/audio encoder 1 and then outputs the coded picture signal toward buffer 7 and the coded audio signal toward buffer 9. Buffer 7 stores temporarily the coded picture signal from demultiplexer 6 and then outputs it toward picture decoder 8, while buffer 9 stores temporarily the coded audio signal from demultiplexer 6 and then outputs it toward audio decoder 10.
The data transmission rate of the combined bit stream outputted from the encoder as shown in FIG. 66 and inputted toward the decoder as shown in FIG. 67 is decided by that of the transmission lines which is employed.
The data transmission rate of the combined data stream is fixed, for example, to 64 kbits/sec in ISDN and 56 kbits/sec in PSTN.
Although variable rate is obtainable in the Internet or ATM (asynchronous transfer mode), the fixed rate is preferable.
It is of importance to distribute the quantity of information or the code quantity for the picture information and audio information, when the fixed rate is employed.
Conventionally, the rate for audio signal is fixed at first, and then the rate for picture signal is fixed.
JP 6-216779 A (1994) discloses a code distribution system, wherein audio encoder notifies the picture buffer of the presence of audio signal. Therefore, when the audio signal is absent, the code quantity allocated to the audio signal is re-allocated to the picture signal, whereby it is avoided that silent audio signal is transmitted in vain, and the quality of picture is improved by the re-allocation.
JP 2-246431 A (1990) discloses a coding and transmission system for audio and visual information, wherein the transmission bandwidth is distributed, according to the presence or absence of the audio signal. Concretely, a clock signal for the bandwidth distribution is generated. The receiver side demultiplexes the audio-visual signal on the basis of the clock.
JP 63-252083 A (1988) discloses a time slot multiplexing system, wherein inputted level of audio data is always monitored in order to utilize the audio presence information which is multiplexed on prescribed time slot. When the audio signal is present, the audio signal and visual signal are multiplexed on the time slots for each signal. On the contrary, when the audio signal is absent, visual signal is multiplexed on the time slot which is once allocated for audio signal.
These three Japanese patent applications relate to a deprivation of the bandwidth for audio signal in case of the absence of audio signal, whereby the picture quality is improved, in place of transmitting silent audio information.
However, the conventional prior art as explained above have a disadvantage that only the fixed length coding is employed for the audio signal. Although it is detected only whether or not audio signal is present in case of the fixed length coding, a variation in the code quantity as well as the presence of audio signal should be taken into account, when the variable length coding is employed.
Further, the conventional prior art has a disadvantage that the quality of audio signal is neglected, because the audio quality varies, even when the code quantity is fixed the fixed length coding system.
Further, the conventional prior art has a disadvantage that the audio quality is apt to be degraded greatly, because the coding format is the same, regardless of the kinds of audio signal such as human voice or music. Therefore, a plurality of coding formats are required.
Further, the conventional prior art has a disadvantage that the picture quality varies, because the allocated code quantity for visual signal is the same every frame. Actually, the code quantity is increased, when picture movements are very active or scenes change very often.
Further, the conventional prior art has a disadvantage that degree of freedom for controlling the qualities of audio/visual signal is strictly restricted, because the code distribution is executed only by detecting the audio signal presence. Accordingly, visual priority control, audio priority control, or audio-visual minimum guarantee is not available.
Therefore, an object of the present invention is to distribute available signal codes adaptively under the variable rate coding, in order to improve the audio-visual quality.
The coding apparatus for picture and audio signals of the present invention comprises: a picture encoder for compressing and coding a picture signal; a rate control means for storing the picture signal coded by the picture encoder and for controlling a quantity of code of the picture signal; a frequency conversion means for converting a samplimg frequency of an audio signal; an audio encoder for compressing and coding the audio signal of which sampling frequency is converted by the frequency conversion means; a code quantity control means for executing a distribution of a quantity of code to the picture signal coded by the picture encoder and the audio signal coded by the audio encoder; and a multiplexer for multiplexing the picture signal coded by the picture encoder and the audio signal coded by the audio encoder. The code quantity control means outputs a result of the distribution toward the rate control means, the frequency conversion means and the audio encoder.
According to the present invention, the variable length audio signal coding which is not adopted by the conventional picture/audio coding equipment becomes available, whereby audio signals can be processed under variable rates.
Concretely, coding formats and coding rates suitable for the kinds of audioignals can be selected.
Further, picture qualities can be maintained by changing the distribution of codes for audio and pictures.
Audio qualities also can be maintained by changing the distribution of codes for audio and pictures.
Therefore, qualities of audio and pictures are controlled adaptively by changing the distribution of codes for audio and pictures.